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Suppose a program contains 10% code to be executed serially.

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Lets assume there is $100$ units of code to be executed and let $1$ unit of code takes $1$ second to execute.

 

Using $4$ processors

$10$ units are executed in $10$ seconds

$90$ units are divided among $4$ processors and executed parallely so the time it takes $= 90/4 = 22.5\approx 23$ seconds.

Time -> 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
Processor 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
Processor 2                     1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
Processor 3                     1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1  
Processor 4                     1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1  

$Effeciency = \frac{Boxes \ used}{Total \ Boxes}=\frac{100}{132} = 0.757$

 

Using $8$ processors

$10$ units are executed in $10$ seconds

$90$ units are divided among $8$ processors and executed parallely so the time it takes $= 90/8=11.25 \approx 12$ seconds.

Time -> 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
Processor 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
Processor 2                     1 1 1 1 1 1 1 1 1 1 1 1
Processor 3                     1 1 1 1 1 1 1 1 1 1 1  
Processor 4                     1 1 1 1 1 1 1 1 1 1 1  
Processor 5                     1 1 1 1 1 1 1 1 1 1 1  
Processor 6                     1 1 1 1 1 1 1 1 1 1 1  
Processor 7                     1 1 1 1   1 1 1 1 1 1  
Processor 8                     1 1 1 1 1 1 1 1 1 1 1  

$Effeciency = \frac{Boxes \ used}{Total \ Boxes}=\frac{100}{176} = 0.568$ 

 

$\therefore$ Option $C.$ is the most appropriate choice.

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